Abstract
Carbon isotope fractionation has been examined in various photosynthetic pathways at the molecular and cellular level, using modern biochemical techniques for the isolation of pure metabolic and cellular constituents. Photosynthetic CO₂ fixation in the Calvin cycle resulted in similar δ¹³C values (-19.4 to -26.6°/οο) for selected metabolic intermediates in two morphologically different organisms: Chromatium, a photosynthetic bacteria; and cotton. The primary isotope effect occurred at the CO₂ fixation step. At the molecular level, CO₂ fixation by ribulose diphosphate carboxylase resulted in a 25°/ οο larger fractionation than has been previously reported (-17.4°/οο compared to -42.9°/οο at 25°C). A decrease in ¹³C of 1°/οο per °C was observed in sugar synthesis in the living cell and the cell free synthesis of PGA. Photosynthetic carbon metabolism in a Hatch-Slack plant demonstrated a 3°/οο isotope fractionation between the CO₂ and organic carbon. Lipid synthesis resulted in a greater isotope fractionation than CO₂ fixation. The cell-free synthesis of malic acid from CO₂ and PEP resulted in a 3°/οο enrichment of ¹²C into malic acid which was in good agreement with fractionation in the living system. The δ¹³C values of the metabolic intermediates in marsh grass and carbonate algae were intermediate between those in the Calvin and Hatch-Slack pathway. Chromatium, when grown heterotrophically on acetate as the carbon source, resulted in an enrichment in ¹²C in the sugars (-18.9°/οο vs source acetate). The lipids were only 7.7°/οο enriched in ¹²C with respect to the acetate. Dark-germinated castor beans, which utilize fat and acetate for the carbon source in sugar synthesis, had a similar isotope composition in the metabolic intermediates.
Whelan, Thomas (1971). Stable carbon isotope fractionation in photosynthetic carbon metabolism. Doctoral dissertation, Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -174131.